Here are a few comments on the document: - The document provides detailed information on estimating evaporation using standard evaporation pans and Colorado sunken pans. It describes the design, installation, and measurement procedures for both types of pans. - Factors that influence evaporation rates are discussed, including temperature, humidity, wind speed, solar radiation, and atmospheric pressure. - Pan coefficients are presented to account for differences in heat transfer between pans and natural water bodies when estimating actual evaporation or evapotranspiration from pan measurements. - Examples of evaporation rate calculations from pan readings over a 24-hour period are shown. - This appears to be an educational document meant to teach students the process and considerations for

2. Experiment #01
To estimate the evaporation by using
standard evaporation pan and Colorado
sunken pan
Hydrology and Water Resource Management
Lab

3.  Objective
To estimate the evaporation by using Standard evaporation pan or Class A Pan and
Colorado Sunken pan.
 To understand the real time evaporation from lakes, streams, and any water body
because evaporation is the important factor in hydrological cycle.
Evaporation.
 Evaporation is the physical process by which water changes from a
liquid phase to a gaseous phase or vapor below the boiling point. This
is due to gaining of energy from sun and depends on different factors.
It is a day night process. It is a net process because water vapor is
constantly moving back and forth from the water surface.

4. Factors affecting evaporation
 Temperature:
Evaporation is highly dependent upon temperature. Higher the
temperature, greater will be the evaporation.
 • Surface area of water body:
Evaporation takes place from free surface of water body. Larger
the area exposed to atmosphere, greater will be evaporation.
 • Humidity:
Greater the humidity, lesser will be the evaporation

5.  Wind speed:
Higher the wind speed more will be evaporation.
 • Solar radiation:
 Evaporation increases with increasing solar radiation.
 • Atmospheric Pressure:
 Evaporation decreases with increasing atmospheric pressure.
 • Depth of Water:
 Greater the depth of water, lesser will be the evaporation.
Factors affecting evaporation

6. Evaporation Estimation
Evaporation can be estimated by different methods but we only discussed two of
them in detail
Evaporation can be estimated by:
Indirect or theoretical method
Water Budget Approach,
Energy Budget Approach, and
Mass Transfer Approach
 –Direct measurement using pans,
The pan is the most widely used evaporation instrument. There are three types of
pans.
 • Sunken pans
 • Above ground or surface pans
 • Floating pans.

7. Standard evaporation pan
Evaporation through Standard pan
The Evaporation pan is circular in shape, 120.7 cm in diameter
and 25 cm deep. It is made of galvanized iron (22 gauges) or
Monel metal (0.8 mm). The pan is mounted on a wooden open
frame platform which is 15 cm above ground level in order to
provide good circulation mechanism of wind. The pan must be
level. . It is filled with water to 5 cm below the rim, and the
water level should not be allowed to drop to more than 7.5 cm
below the rim. . Pans should be protected by fences to keep
animals from drinking. Pan readings are taken daily in the early
morning at the same time that precipitation is measured.
Measurements are made in a stilling well that is situated in the
pan near one edge. The stilling well is a metal cylinder of about
10 cm in diameter and some 20 cm deep with a small hole at the
bottom. Evaporation measurement should be taken in the stilling
well accurately by vernier gauge.
Important point= NICKEL AND COPPER
ALLOY (MONEL)

8. Evaporation through Sunken pan
Sunken pans are of three types.
 Bureau of Plant Industry ( B. P. I. ) Pan
This pan is circular in shape. These are 1.80 m in diameter and 0.60 m deep. These are
sunken into the ground to a depth of 0.55 m and 0.05 m above the ground i.e. total depth is
0.60 m.
 Young’s Pan
It is also circular in shape, about 0.60 m in diameter and 0.90 m deep covered with a 6
mm wire mesh screen. Other conditions are same as that of Colorado pan.
 Colorado Pan

9. Colorado Sunken pan
Colorado Sunken pan
 The Colorado sunken pan is 92 cm, square in shape and 46
cm deep, made of 3 mm thick iron, placed in the ground
with the rim 5 cm (2 in) above the soil level. The pan is
painted with black tar paint. Black tar is anti-corrosive.
The water level is maintained at or slightly below ground
level, i.e., 5-7.5 cm below the rim.
 It have cylindrical stilling well and measurement of
evaporation is taken with help of vernier point gauge.
 Measurements are taken similarly to those for the pan.
Siting and environment requirements are also similar to
those for the Class A pan.
 This type of pan is mostly resemblance to our real water
bodies like lakes and seas. . The disadvantage is that
maintenance is more difficult and leaks are not visible

10. Procedure (Standard Pan)
 The pan is installed in the field.
 The pan is filled with a known quantity of water (the surface area of
the pan is known and the water depth is measured).
 The water is allowed to evaporate during a certain period of time
(usually 24 hours). For example, each morning at 7 o’clock a
measurement is taken. The rainfall, if any, is measured
simultaneously.
 After 24 hours, the remaining quantity of water (i.e. water depth) is
measured.
 The amount of evaporation per unit time(the difference between the
two measured water depths) is calculated; this is the pan
evaporation: E pan (in mm/24 hours).
 Unit is mm/24hrs.

11. Draw back of standard pan
 The pans does not give the exact measure of large reservoir with
following drawbacks
 They differ in heat storing capacity
 Heat transferred from sides and bottom
 Height of rim in evaporation pan affects the wind action over the
surface
 The rim height also casts a shadow of variable magnitude over
surface of water
 Heat transfer characteristics of pan material is different from that of
the reservoir.
 Due to these reasons a correction factor is required

12. Pan Coefficient
 Pan Coefficient
 The correction factor is given as
 Lake Evaporation = Cp x Pan evaporation
 Pan coefficient values are given as
 Sr. No Type of Pan Average Value Range
Class A Land type 0.70 0.60‐0.80
(Modified Class A) 0.80 0.65‐1.1
Coloroda Sunken Pan 0.78 0.75‐0.86
USGS Floating Pan 0.80 0.70‐0.82

13. Evapotranspiration
 Transpiration is a biological process in which the water
molecules are lost in the form of water vapours from
the aerial parts of the plants. This natural phenomenon
is carried in all green plants, green crops and grass. It
is a day time process.
Evapotranspiration
Evapotranspiration (ET) is a term used to refer to the
combined processes by which water moves from the
earth’s surface into the atmosphere. It covers both
water evaporation (movement of water to the air directly
from soil, and water bodies) and transpiration (movement
of water from the soil, through roots and bodies of
vegetation, and then into the air). We can also estimate
evapotranspiration in the term of standard evaporation pan
value by introducing pan coefficient which depends on
different factors .
The water in the pan and the grass do not react in exactly
the same way to the climate.

14. Evapotranspiration
 The pan coefficient, K pan, depends on:
 the type of pan used.
 the pan environment: if the pan is placed in a fallow or cropped area
 the climate: the humidity and wind speed.
 For evaporation pan, the K pan varies between 0.35 and 0.85.
 Average K pan = 0.70.
 If the pan factor is not known the average value could be used.
 Formula
 ETо = K pan × E pan
ETо: reference crop evapotranspiration
K pan: pan coefficient
 E pan: pan evaporation

15. Class A pan Case A: placed in short green cropped area CASE B: Pan placed in dry fallow area
RH mean (%) Low < 40 Medium 40-
70
High > 70 Low < 40 Medium 40-
70
High > 70
Wind speed Windward
side distance
of green crop
Windward
side distance
of dry fallow
(𝐦𝐬−𝟏
) (m) (m)
Light 1 .55 .65 .75 1 .7 .8 .85
<2 10 .65 .75 .85 10 .6 .7 .8
100 .7 .8 .85 100 .55 .65 .75
1000 .75 .85 .85 1000 .5 .6 .7
Moderate 1 .5 .6 .65 1 .65 .75 .8
2-5 10 .6 .7 .75 10 .55 .65 .7
100 .65 .75 .8 100 .5 .6 .65
1000 .7 .8 .8 1000 .45 .55 .6
Strong 1 .45 .5 .6 1 .6 .65 .7
5-8 10 .55 .6 .65 10 .5 .55 .65
100 .6 .65 .7 100 .45 .5 .6
1000 .65 .7 .75 1000 .4 .45 .55
Very strong 1 .4 .45 .5 1 .5 .6 .65
>8 10 .45 .55 .6 10 .45 .5 .55
100 .5 .6 .65 100 .4 .45 .5
1000 .55 .6 .65 1000 .35 .4 .45
Table 1 : Pan Coefficients (Kp) for Standard pan for different pan siting and
environment and different levels of mean relative humidity and wind speed.

16. Table 2 Pan Coefficients (Kp) for Colorado sunken pan for different pan siting
and environment and different levels of mean relative humidity and wind speed.
Sunken
Colorado
Case A: placed in short green cropped area CASE B: Pan placed in dry fallow area
RH mean (%) Low < 40 Medium
40-70
High > 70 Low < 40 Medium
40-70
High >
70
Wind speed Windward
side distance
of green crop
Windward
side distance
of dry fallow
(𝐦𝐬−𝟏
) (m) (m)
Light 1 .75 .75 .8 1 1.1 1.1 1.1
<2 10 1.0 1.0 1.0 10 .85 .85 .85
≥100 1.1 1.1 1.1 100 .75 .75 .8
1000 .7 .7 .75
Moderate 1 .65 .7 .7 1 .95 .95 .95
2-5 10 .85 .85 .9 10 .75 .75 .75
≥100 .95 .95 .95 100 .65 .65 .7
1000 .6 .6 .65
Strong 1 .55 .6 .65 1 .8 .8 .8
5-8 10 .75 .75 .75 10 .65 .65 .65
≥100 .8 .8 .8 100 .55 .6 .65
1000 .5 .55 .6
Very strong 1 .5 .55 .6 1 .7 .75 .75
>8 10 .65 .7 .7 10 .55 .6 .65
≥100 .7 .75 .75 100 .5 .55 .6
1000 .45 .5 .55

17. Calculations and Observations
Sr. No Water depth/level
before
Evaporation
Water depth/level
after
Evaporation
Evaporation
mm/24hrs
ETо = K pan × E
pan
1 20 cm 18cm 2cm/24hrs ETo=0.7*2 = 1.4 cm/24hrs
2 13+12*0.05= 13.6mm 18+0.05*8 =18.4mm 4.8mm/24hrs Eto=.7*4.8=3.36 mm/24hrs

18. WRITE YOUR OWN COMMENTS